user2629828 user2629828 - 2 months ago 14
Android Question

Android - Null pointer Exception - findViewById()

Can anyone help me to find out what can be the issue with this program.
In the

onCreate()
method the
findViewById()
returns null for all ids and this causes a null pointer exception later. I can not figure out why the
findViewById()
can not find the view. Any suggestions?

This is the main code:

public class MainActivity extends Activity {

ViewPager pager;
MyPagerAdapter adapter;
LinearLayout layout1, layout2, layout3;

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

layout1 = (LinearLayout) findViewById(R.id.first_View);
layout2 = (LinearLayout) findViewById(R.id.second_View);
layout3 = (LinearLayout) findViewById(R.id.third_View);

adapter = new MyPagerAdapter();
pager = (ViewPager) findViewById(R.id.main_pager);
pager.setAdapter(adapter);
}

private class MyPagerAdapter extends PagerAdapter
{

@Override
public int getCount() {
return 3;
}

@Override
public Object instantiateItem(ViewGroup collection, int position) {

LinearLayout l = null;

if (position == 0 )
{
l = layout1;
}
if (position == 1)
{
l = layout2;
}

if (position == 2)
{
l = layout3;
}
collection.addView(l, position);
return l;
}

@Override
public boolean isViewFromObject(View view, Object object) {
return (view==object);
}

@Override
public void destroyItem(ViewGroup collection, int position, Object view) {
collection.removeView((View) view);
}
}
}


And the related XML files:

activity_main layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:background="#a4c639">


<android.support.v4.view.ViewPager
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/main_pager"/>
</LinearLayout>


activity_first layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/first_View">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />

<Button
android:id="@+id/button1"
style="?android:attr/buttonStyleSmall"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Button" />

</LinearLayout>


activity_second layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/second_View">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />

</LinearLayout>


And the activity_third layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/third_View">

<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/hello_world" />

</LinearLayout>

Answer

findViewById() returns a View if it exists in the layout you provided in setContentView(), otherwise it returns null and that's what happening to you.

Example if you setContentView(R.layout.activity_first); and then call findViewById(R.id.first_View); it will return a View which is your layout.

But if you call findViewById(R.id.second_View); it will return null since there is not a view in your activity_first.xml layout called @+id/second_View.